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Journal of Minerals & Materials Characterization & Engineering, Vol. 9, No.3, pp.263-274, 2010
jmmce.org Printed in the USA. All rights reserved
Microstructural Study of Heat Treated Chromium Alloyed Grey Cast Iron
, A. Okeowo
and O. Oluwole
Materials Science and Engineering Department, Obafemi Awolowo University
Mechanical Engineering Dept, University of Ibadan
*Corresponding Author: email@example.com
This work investigated the effect of annealing, normalizing and quenching heat treatments on the
microstructure of some chromium alloyed grey cast iron.
Three sets of ten samples each having chromium contents of 0.5, 1.5 and 2.5% were heat treated
above the upper critical temperature, to austenitizing temperatures (800
one hour each and then annealed, normalized and water quenched. Metallographic analyses of
the heat treated samples were done. The results showed carbides content increased with
increasing Chromium addition.
Key words: Microstructure; Grey cast iron; Chromium alloying; Heat-Treatment
Gray iron is the most widely used, with annual production several times total of all other cast metals. It
has excellent machinability, good wear resistance, and high vibration absorption
. Gray iron is valued
particularly for its ability to be cast into complex shapes at relatively low cost. Thus, its application
includes: Sanitary wares, household appliances, rolling mill and general machinery parts, ingot moulds,
cylinder blocks and heads for I.C. engines, frames for electric motors, machine tool structures, etc.
Chromium up to 1 percent increases hardness, strength and depth of chill
. Higher percentage of
chromium makes cast iron resistant to corrosion and high temperature
. Chromium occurs as a
carbide in iron and increases the tendency for the formation of white iron during solidification
and the retention of a higher combined carbon in the solid state
. This increases the wear
resistance of the iron
264 P. Atanda, A. Okeowo and O. Oluwole Vol.9, No.3
The heat treatment of grey irons can considerably alter the matrix microstructure with little or no
effect on the size and shape of the graphite achieved during casting
. The matrix microstructures
resulting from heat treatment can vary from ferrite-pearlite to tempered martensite.
The annealing of gray iron consists of heating the iron to a temperature high enough to soften it
and/or to minimize or eliminate massive eutectic carbides, thereby improving its machinability.
This heat treatment reduces mechanical properties substantially
. It reduces the grade level
approximately to the next lower grade: for example, the properties of a class 40 gray iron will be
diminished to those of a class 30 gray iron. The graphite morphology does not change to any
significant extent during normalization. Higher normalizing temperatures increase the carbon
solubility in austenite (that is, the cementite volume in the resultant pearlite). A higher cementite
volume, in turn, increases both the hardness and the tensile strength
Gray irons are hardened and tempered to improve their mechanical properties, particularly
strength and wear resistance. After being hardened and tempered, these irons usually exhibit
wear resistance approximately five times greater than that of pearlitic gray irons
2. MATERIALS AND METHOD
The samples needed for the research work were obtained from Alarat Engineering Company,
Osogbo, Nigeria. The samples were basically that of unalloyed grey cast iron and alloyed grey
cast iron of different compositions with chromium, i.e. 0.5%Cr, 1.5%Cr and 2.5% Cr. The
spectrometric analyses are presented in Table 1.
Table 1. Chemical Composition of Cr alloyed Cast- Iron
Alloy Type C Si S P Mg Cu
Un-alloyed Cast Iron 3.50
0.5% Cr alloyed Cast Iron
1.5% Cr alloyed Cast Iron
2.5% Cr alloyed Cast Iron
2.2.1 Specimen preparation
The specimens for metallographic test, heat treatment, hardness test, chemical compositional
analysis, and micro structural analysis were cut from the obtained samples.
Vol.9, No.3 Microstructural Study of Heat Treated Chromium alloyed Grey Cast Iron 265
This operation was aimed at producing a surface that is flat, smooth, and free from surface
contaminations. The specimen was ground on a series of silicon carbide papers of increasing
fineness. The papers were abrasive silica carbide papers with ranges of 240, 320, 400 and 600
grits were used sequentially. This grading enabled the specimen to be ground using coarser paper
first, which allowed for a more effective removal of the surface contaminants and as the
specimen moved from one grade of cloth to the other, it is rotated through 90
to allow for the
removal of scratch lines done on the surface by the first grade of grinding cloth. This process
was well flushed with running water to wash away the debris/metal particles which was capable
of being embedded at the surface of the specimen. Each sample was thoroughly cleaned after
each grinding step to avoid any left over of abrasive particles carried over to the next fine
abrasive grinding step. This procedure was continued until a fine, flat and smooth surface was
Fine scratches were removed from the surface of the specimen with the aid of a mechanical
polishing machine, metasor
universal polishing machine. The surface of the specimen was held
lightly on a horizontally rotating polishing disc covered with “Selvyt” cloth. Liquid abrasive
compound and polishing agents, diamond dust were applied to the selvyt cloth while the
specimens were being polished. By careful adjustment of hand pressure and by using
progressively finer abrasives, a highly polished surface i.e. mirror like shining surfaces free from
scratches were produced.
After polishing, the specimen was washed with water and Natal etchant was used.
The surface of the specimen was observed using a computer aided optical microscope
(Accuscope) at the Materials Science and Engineering Department laboratory. Micrographs were
obtained by using Fametech camera with DCM 350k pixels USB.
2.2.6 Heat treatment
In this research work, the as cast samples (unalloyed and chromium alloyed) were heat treated in
the furnace above A
into the austenitic region (Upper critical temperature). The temperatures
that were employed in this work were; 800
C and 900
C. Each of the specimens were
266 P. Atanda, A. Okeowo and O. Oluwole Vol.9, No.3
heated in the furnace at the said temperatures and subjected to different heat treatment
operations. A set was annealed; another normalized, and the last quenched in cold water.
2.2.7 Hardness test
In this work, Brinell hardness measurements were used for hardness tests.
BHN = 2F
Where F = Applied load in Kg
D = Diameter of indenter in (mm)
d = Diameter of indentation in (mm)
3. RESULTS AND DISCUSIONS
The micrographs of the heat treated cast irons are presented in Figures 1- 10. Table 2 shows the
Brinnell hardness values obtained for quenched un-alloyed and cast-iron alloys which present
Figures 1a- d show the micrographs of the as-cast samples. Increasing carbide formation is
observed as chromium content increased from 0.5 to 2.5%. Figures 2a-d show the micrographs
of samples annealed at 800
C. Figures 3a- d shows the micrographs of samples normalized at
C. The increasing carbide content with increasing Chromium content is very apparent.
Figures 4a-d show the micrographs of samples water quenched at 800
C. The micrographs of
samples annealed at 850
C are presented in Figures 5a-d. The un-alloyed sample is seen to have
high free carbide content in the cast –iron matrix. Figures 6a-d show the micrographs of samples
normalized at 850
C and Figures 7a-d presents the micrographs of samples water quenched at
C. Figures 8a-d show the micrographs of samples annealed at 900
C; Figures 9a-d the
micrographs of samples normalized at 900
C and Figures 10a-d the micrographs of samples
water quenched at 900
The micrographs show a general trend of increasing amount of carbides with increasing
chromium addition. Also, there is the general trend of increasing carbide amount with increasing
temperature of heat treatment.
Vol.9, No.3 Microstructural Study of Heat Treated Chromium alloyed Grey Cast Iron 267
(a)As cast; not alloyed with Cr (b) As cast 0.5% Cr
(c)As cast 1.5% Cr (d)As cast 2.5% Cr
Figs. 1a-d. Micrographs of as cast specimens (X 200).
(a) Unalloyed, annealed at 800
C (b) 0.5% Cr, annealed at 800
(c)1.5% Cr, annealed at 800
C (d) 2.5% Cr, annealed at 800
Figs.2a-d. Micrographs of annealed specimens at 800
268 P. Atanda, A. Okeowo and O. Oluwole Vol.9, No.3
(a)Unalloyed, normalized at 800
C (b) 0.5% Cr, normalized at 800
(c)1.5% Cr, normalized at 800
C (d)2.5% Cr, normalized at 800
Figs. 3a-d. Micrographs of normalized specimens at 800
(a) Unalloyed, quenched at 800
C (b) 0.5% Cr, quenched at 800
(c) 1.5% Cr, quenched at 800
C (d) 2.5% Cr, quenched at 800
Figs. 4a-d. Micrographs of water quenched specimens at 800
C (X 200).
Vol.9, No.3 Microstructural Study of Heat Treated Chromium alloyed Grey Cast Iron 269
(a) Unalloyed, annealed at 850
C (b)0.5% Cr, annealed at 850
(c) 1.5% Cr, annealed at 850
C (d) 2.5% Cr, annealed at 850
Figs. 5a-d. Micrographs of annealed samples at 850
C (X 200).
(a) Unalloyed, normalized at 850
C (b) 0.5% Cr, normalized at 850
(c) 1.5% Cr, normalized at 850
C (d) 2.5% Cr, normalized at 850
Figs. 6a-d. Micrographs of normalized samples at 850
C (X 200).
270 P. Atanda, A. Okeowo and O. Oluwole Vol.9, No.3
(a) Unalloyed, quenched at 850
c (b) 0.5 % Cr, quenched at 850
(c) 1.5% Cr, quenched at 850
C (d) 2.5% Cr, quenched at 850
Figs.7a-d. Micrographs of water quenched samples at 850
C (X 200).
(a) Unalloyed, annealed at 900
C (b) 0.5 % Cr, annealed at 900
(c) 1.5 % Cr, annealed at 900
C (d) 2.5% Cr, annealed at 900
Figs. 8a-d. Micrographs of annealed specimens at 900
C (X 200).
Vol.9, No.3 Microstructural Study of Heat Treated Chromium alloyed Grey Cast Iron 271
(a) Unalloyed, normalized at 900
C (b).5% Cr, normalized at 900
(c) 1.5% Cr, normalized at 900
C (d) 2.5% Cr, normalized at 900
Figs. 9a-d. Micrographs of normalized specimens at 900
C (X 200).
(a) Unalloyed, quenched at 900
C (b)0.5% Cr, quenched at 900
(c) 1.5 % Cr, quenched at 900
C (d) 2.5% Cr, quenched at 900
Figs. 10a-d. Micrographs of water quenched specimens at 900
272 P. Atanda, A. Okeowo and O. Oluwole Vol.9, No.3
The Brinell hardness values of the quenched un-alloyed and the cast –iron alloys are presented in
Table 2. A general trend of increasing hardness with increasing chromium content was observed.
Table 2. Brinell hardness numbers (BHN) for quenched specimens.
As cast quenched form 800
As cast quenched from 850
As cast quenched from 900
0.5%Cr quenched form 800
1.5%Cr quenched from 800
2.5%Cr quenched from 800
0.5%Cr quenched form 850
1.5%Cr quenched from 850
2.5%Cr quenched from 850
0.5%Cr quenched from 900
1.5%Cr quenched from 900
2.5%Cr quenched from 900
From the micrograph of the as received sample, Fig.1a, it was observed that graphite flakes are
very prominent having long morphology. However, in the chromium alloyed samples (Figs. 1b-
d), increasing carbide formation with increasing Cr addition was easily observed; free graphite
In the 800
C annealed samples (Figs.2a-d), there was a gradual increase of carbide precipitation
from the unalloyed sample to 2.5% Cr sample as the samples had ample time for the carbides to
form. A coarse grained structure easily machinable is formed. On the other hand, in the
normalized material, the carbides did not fully form because of faster cooling (Figs.3a-d).
Hardening produced very brittle martensitic structure. Due to the drastic quenching medium, the
samples quenched in water did not have the carbides fully precipitating out. If a tempering action
followed, a change in carbide structure would have been visible (Fig.4a-d).
It was observed from the heat treatment at 850
C that: Graphitization is more evident especially
in the annealed samples (Figs.5a-d) and coarser carbide structures compared to 800
observed in the normalized samples (Figs.6a-d) because of the higher normalizing temperature.
Vol.9, No.3 Microstructural Study of Heat Treated Chromium alloyed Grey Cast Iron 273
The quenched samples (Figs. 7a-d) structures followed the trend of the quenching at 800
with more massive carbide held in the matrix.
Heat treatment at 900
C gave rise to the precipitation of coarse brittle Cr-carbides (Figs. 8-10).
In the normalized samples, higher normalizing temperatures increased the carbon solubility in
austenite, that is the cementite volume in the resultant pearlite. A higher cementite volume, in
turn, increases the hardness. Chromium generally tends to become brittle during heat treatment
because chromium promotes grain growth. At this temperature (900
C) grain growth results.
This is because the higher the temperature the coarser the grain structures and thus grain growth
can occur. In all quenched steels, varying amounts of austenite may be retained. Such retained
austenite will give rise to soft spots but can be made to transform to martensite by further
From the micrographs, generally it could be seen that chromium joins with carbon to form
chromium carbide, thus adding to depth hardenability.
3.2.2 Discussion on the Hardness Results
From the hardness test results, it was observed that increasing chromium content increased
hardness in the quenched samples for unalloyed and 0.5% Cr alloyed grey iron. However, for
1.5% and 2.5% Cr alloyed grey irons, it was observed that the highest hardness for the three
alloys was obtained from water quenching at 850
From the chemical composition of the samples, it was evident that this material fitted the ASTM
A – 159-77-SAE specification J 431 –Grade 30 categories, for automotive gray iron castings.
Considering the micrograph results with values obtained from the hardness tests it could be said
that chromium forms chromium carbides, the reason for its depth hardenability and resistance to
abrasion and wear.
In addition to Cr-carbide formation, it could also be said that hardness increased generally with
increasing chromium addition. The highest hardness value was obtained at 2.5% Cr addition, for
the sample water quenched from 850
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